1. Field of the Invention
The invention relates to a thermally powered rotary actuator which is used for positioning a rotatable shaft in first and second positions which are disposed 180.degree. apart. More particularly, the invention relates to a thermally powered rotary actuator which is used for positioning a magnetometer in two positions disposed 180.degree. apart for calibration purposes.
A magnetometer is an apparatus which is used for measuring the strength of a magnetic field. The magnetometer comprises a core of magnetic material which is surrounded by a gating coil and a pickup coil. The gating coil is driven by an alternating current source which periodically saturates and desaturates the magnetic core. The lines of flux of the ambient magnetic field passing through the core change the degree of saturation of the core produced by the magnetic field of the gating coil. Accordingly, the flux of the ambient magnetic field passing through the core induces a voltage which differs from the voltage induced by the gating coil as a function of the strength of the ambient magnetic field.
Magnetometers are sensitive to magnetic fields along the axis of the magnetic core. A magnetometer is malfunctioning when different readings of the strength of the ambient magnetic field are obtained for positions of the core which are disposed 180.degree. apart. When different readings have been obtained of the strength of the ambient magnetic field when the core is disposed 180.degree. apart, the magnetometer must be recaliberated by introducing a correction factor to ensure proper operation. The correction factor is obtained by subtracting the reading given at the two core positions disposed 180.degree. apart and dividing this quantity by 2 and then adding this quantity to the reading taken at the low reading position or subtracting it from the reading which is taken at the high reading position.
With the inception of interplanetary space exploration by unmanned spacecraft, it has become extremely critical to have a rotary actuator which will flip a magnetometer between first and second positions which are disposed 180.degree. apart to permit remote calibration of the magnetometer.
2. Description of the Prior Art
Thermally powered actuators used for rotating a shaft through angular displacements are well known. These actuators generally comprise one or more bimetallic springs which are wound in a convoluted configuration and which are attached at their respective ends to the rotatable shaft and to a fixed point on a frame which rotatably supports the shaft. Application of heat to one of the bimetallic springs causes extension of the spring which rotates the shaft through an angular displacement. Thermal actuators of this type differ substantially from the present invention in that they do not include a detent mechanism for locking the shaft in first and second rotary positions which are disposed 180.degree. apart. Moreover, the heat extensible springs used in these thermally powered motors are not constructed from an alloy possessing mechanical memory such as 55-Nitinol. U.S. Pat. Nos. 2,323,738, 2,340,170, 3,423,608, 3,446,998 and 3,663,839 each disclose thermally powered actuators of the aforementioned type which rotate a shaft through an angular displacement.